EPR

Question 1

How are EPR and NMR similar?

Answer 1

Both use less energetic photons and must be observed in a magnetic field. Both observe spin transitions.

Question 2

What are the major differences between EPR and NMR?

Answer 2

Microwaves are used in EPR and radiowaves are used in NMR. EPR observes electron spin transitions and NMR observes nuclear spin transitions.

Question 3

What does EPR measure?

Answer 3

The energy difference between the two states of a paramagnet when in a magnetic field.

Question 4

Is an energy difference between states when the molecule has not been placed in a magnetic field?

Answer 4

No

Question 5

Which has a higher magnetic moment, the unpaired electron spin in EPR or the unpaired nuclear spin in NMR?

Answer 5

Electron spin in EPR

Question 6

What is a paramagnet?

Answer 6

An atom or molecule with an unpaired electron spin.

Question 7

What is the zeeman effect?

Answer 7

When in a magnetic field, the magnetic moment of the paramagnet will either align with or against the magnetic field.

Question 8

What is the highest energy state of a paramagnet?

Answer 8

When the paramagnet aligns against the magnetic field.

Question 9

Describe most paramagnets in the ground state at low temperatures.

Answer 9

Aligned with the magnetic field (low energy state)

Question 10

What is the result of using a bigger magnetic field?

Answer 10

More energy is required to make the transition between the two states.

Question 11

Why are microwaves used in EPR?

Answer 11

The energy gap between the two electron spin states corresponds to microwaves in the GHz range.

Question 12

In EPR, which is kept constant the microwave frequency or the magnetic field?

Answer 12

Microwave frequency

Question 13

When does absorption occur?

Answer 13

At the resonance position - where ΔE matches the energy of the microwaves

Question 14

What is the advantage of scanning the magnetic field rather than scanning the microwave frequency?

Answer 14

It is easier to scan the magnetic field. This gives a better signal:noise ratio and gives more control of the system.

Question 15

At what frequency is the field modulation in EPR?

Answer 15

100 kHz for measurements at X band microwave frequency

Question 16

What is the advantage of field modulation?

Answer 16

Gives better signal:noise, overcomes wide EPR signals by giving the first derivative graph.

Question 17

What does the zero point on the first derivative graph?

Answer 17

The absorption peak when there is no field modulation.

Question 18

How do most EPR instruments generate microwaves?

Answer 18

Using a gun diode

Question 19

Where is the sample loaded?

Answer 19

In the resonant cavity between the two magnets

Question 20

How are microwaves guided through the EPR instrument?

Answer 20

Using waveguides - open air, brass rectangular channels with dimensions corresponding to the X band wavelength (3cm)

Question 21

Why is liquid helium used?

Answer 21

To keep samples cool and to allow slower relaxation of the sample.

Question 22

What is the purpose of the circulator?

Answer 22

Directs generated microwaves from the microwave source to the resonant cavity and directs reflected radiation to the detector.

Question 23

When is it particularly important to use liquid helium and why?

Answer 23

When studying metalloproteins as the excited state of a metalloprotein relaxes very quickly, giving broad peaks.

Question 24

Describe X band microwave radiation.

Answer 24

Microwaves resonating at a wavelength of 3cm and a frequency of 9.4 GHz.

Question 25

What happens if the frequency of microwave radiation is increased?

Answer 25

Resonance will be seen at higher resolution but this requires a stronger magnetic field.

Question 26

What are the sources of unpaired electrons in biology?

Answer 26

Transition metals, free radicals and triplet states.

Question 27

What is a triplet state?

Answer 27

An atom or molecule with two unpaired electrons.

Question 28

How can integer spin be detected in EPR?

Answer 28

Using parallel mode EPR - where the modulating field is parallel to the applied magnetic field

Question 29

Describe the spectra given by radicals.

Answer 29

Sharp, defined features with narrow bands. Tend to have spin 1/2 and are found around g=2.002

Question 30

Can radicals be detected at room temperature?

Answer 30

Yes, as they are slow relaxing.